1. Field of the Invention
This invention is in the field of cooling biological tissues to very low temperatures, for treatment of medical conditions, as in cryosurgery.
2. Background Information
It is desirable to be able to selectively cool miniature discrete portions of biological tissue to very low temperatures in the performance of cryosurgery, without substantially cooling adjacent tissues of the organ. Cryosurgery has become an important procedure in medical, dental, and veterinary fields. Particular success has been experienced in the specialties of gynecology and dermatology. Other specialties, such as neurosurgery and urology, could also benefit from the implementation of cryosurgical techniques, but this has only occurred in a limited way. Unfortunately, currently known cryosurgical instruments have several limitations which make their use difficult or impossible in some such fields. Specifically, known systems can not achieve the necessary temperature and cooling power to optimally perform cryosurgical ablation, such as in cardiac ablation to correct arrhythmia.
In the performance of cryosurgery, it is typical to use a cryosurgical application system designed to suitably freeze the target tissue, thereby destroying diseased or degenerated cells in the tissue. The abnormal cells to be destroyed are often surrounded by healthy tissue which must be left uninjured. The particular probe, catheter, or other applicator used in a given application is therefore designed with the optimum shape, size, and flexibility or rigidity for the application, to achieve this selective freezing of tissue. Where a probe or catheter is used, the remainder of the refrigeration system must be designed to provide adequate cooling, which involves lowering the operative portion of the probe to a desired temperature, and having sufficient power or capacity to maintain the desired temperature for a given heat load. The entire system must be designed to place the operative portion of the probe or catheter at the location of the tissue to be frozen, without having any undesirable effect on other organs or systems.
It is an object of the present invention to provide a method and apparatus for precooling a primary loop high pressure refrigerant to a point below its critical temperature, to liquefy the primary refrigerant, with a secondary loop refrigeration cycle. This allows the use of a liquid primary refrigerant having a critical temperature below the operating room temperature, in order to achieve the lower temperature possible with such a primary refrigerant.